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```Appendix L                               763

CHAPTER  IX.

P. 481. The Lever-loaded Safety Valve. The effect
of the weights of lever and valve may be taken into account as
a downward force at the valve centre, which has to be overcome
by the steam pressure. In addition to the lengths shewn in
Fig. 438 (measured in inches), and the weight w (in Ibs.), Let

w = weight of lever, in Ibs.

v = weight of valve and valve centre, in Ibs,

/= distance from fulcrum to centre of gravity of lever, in inches.

/ = pressure per square inch of the steam.

d= diameter of valve, in inches.

Then,        Upward moments    =    downward moments

A

P. 487. The Quadric Chain. Referring to Fig. 449,
the examples at 3 and 4 are spoken of as parallel-crank chains ;
while that at i is called a lever-crank chain, from the fact that
the beam rocks like a "simple lever, and the crank completely
rotates.

P. 503. Oldham's Coupling, Fig. 474, is derived from
the elliptic trammels, Fig. 448, where the trammel bar is fixed and
the cross revolves. The relative motion being unchanged, a fixed
pencil held against C's face will produce an elliptical arc. Also
C's centre will describe a circle passing through centres A and B
(Euclid, III. 31), for the angle between grooves is always a right
angle. Hence (Euclid, I. 32) the angular movements of A and B
are identical.

P. 312. Approach and Recess. During approach the
flank of the driving wheel is acting on the point of the driven
wheel (see Fig. 501), while during recess the point of the driver
acts upon the flank of the driven wheel. On the supposition that
the pushing friction during approach is more prejudicial than the```